1. Technical Field of Invention
The invention relates to fragrance compositions to be incorporated into the core of core shell type microcapsules to control the delivery and release of fragrance and optionally other benefit agents, when used as components within liquid household, laundry, personal care and cosmetic products.
The invention further relates to the use of these microcapsules in liquid consumer products, especially household cleaners, laundry products, and personal care, and cosmetic products including thickened and shear thinning liquids which may appear to be gels under conditions of low shear.
2. Background of the Invention
It is known to encapsulate water insoluble fragrances or other non-fragrance materials in small capsules, often termed microcapsules, typically having a diameter between 1 and 1000 micrometers (microns), for a variety of reasons relating to the protection, delivery and release of the fragrance or other materials.
The preparation of microcapsules is described in Kirk Othmer's Encyclopaedia of Chemical Technology 5th edition and also in the following patents U.S. Pat. No. 2,800,457, U.S. Pat. No. 3,415,758 and U.S. Pat. No. 6,261,483 and references therein. One type of microcapsules, referred to as a wall or shell or core shell microcapsule, comprises a generally spherical shell of water- and oil-insoluble materials, typically a network polymer material, within which fragrance or other material is contained.
When such capsules are incorporated in liquid consumer products, e.g. personal care products such as shampoos, hair conditioners, body washes, or shower gels, laundry products such as fabric conditioners or liquid laundry detergents, or household cleaners such as kitchen surface cleaners, problems can arise, with the microcapsules either creaming (rising to the surface) or settling over time, especially if the product is stored. The creaming or settling is due to differences in density between the microcapsule, its contents and the surrounding liquid. Many liquid household cleaners, liquid laundry products and personal care and cosmetic products have densities around 1.00 g per cubic centimeter (g/cm3), while many organic compounds have densities lower than 1.00 g/cm3. So a microcapsule containing a high proportion of fragrance oils or other hydrophobic oils may have a lower density than the liquid phase of the product in which the microcapsules are dispersed, hence these microcapsules will tend to rise or cream over time.
From known physical laws it is possible to calculate a theoretical maximum size for a capsule to remain stably dispersed within a liquid.
Below is an equation for the rate of creaming/settling of an emulsion derived from Stokes law:
  v  =            2      ⁢                        a          2                ⁡                  (                                    d              o                        -            d                    )                    ⁢      g              9      ⁢      η      
v=velocity of creaming/settling;
a=the particle radius;
d0=the density of the continuous phase;
d=the density of the dispersed phase (microcapsule);
g=the gravitational constant;
η=the viscosity of the continuous phase assuming Newtonian shear.
It should be noted that this equation assumes the particles are spherical, uniform and not flocculated. More complex equations but essentially of the same form can be derived for particles of mixed sizes. Also this equation does not include any effects due to Brownian motion which will keep a very small particle dispersed. Despite describing an ideal model and thus not exact for real samples the equation sets out the important factors which govern particle creaming or settling.
It may not be possible or desirable to prepare microcapsules of different (usually smaller) size to reduce creaming or settling as this may have other consequences, such as affecting the ease of breaking the walls for those microcapsules which rely on friability for content release. Moreover less material is encapsulated into a smaller microcapsule requiring a higher proportion of wall material relative to content and a larger number of microcapsules to contain the same amount of core material which consequently may affect product attributes such as colour and importantly cost of manufacture. It may also be undesirable to increase the viscosity of the liquid product in which the microcapsules are dispersed hence it is preferred if the densities of the microcapsules and liquid phase can be more equally balanced.
US patent application 2005/112,152 describes adding solvents to core shell encapsulated fragrances but stipulates that the fragrances must have ClogP greater than 3.3, preferably greater than 8. It is clear from the context that density was not a consideration in selecting these materials since the majority of those named fragrances have densities lower than 1.0 g/cm3 and the higher ClogP requirements suggest that larger alkyl groups are preferred such as glyceryl tributyrate rather than glyceryl triacetate although the latter would be preferred as a density raising ingredient.
EP patent application 1502646 describes density modifiers for co-acervate microcapsules for detergent liquid products but only describes materials which lower the density of the microcapsule.
International application WO 00/59616 also describes modifying the microcapsule contents in order to balance the density with the surrounding liquid. However the materials suggested for raising the density are not very suitable for personal care, laundry and household products being some high density salts or high density hydrophobic liquids such as those containing halogens. It is not desirable or in many cases permissible to include halogenated organic compounds into microcapsules intended for domestic consumer products since many such compounds are believed to have adverse effects on the environment and/or on health.
Surprisingly few organic compounds have densities greater than 0.950 g/cm3 even fewer have densities greater than 1.00 g/cm3 especially if all organic compounds containing halogen atoms are excluded. Compounds having higher densities tend to comprise a substantial proportion of oxygen, nitrogen and sulphur atoms in their molecular formulae and/or possess rings such as aromatic rings in their chemical structures. However such compounds are often quite hydrophilic owing to the polar nature of many functional groups containing oxygen, nitrogen or sulphur atoms. Consequently such hydrophilic compounds may not be efficiently encapsulated by emulsion polymerization techniques.
A further requirement is that encapsulated molecules should not leak from the microcapsules during storage and it has been observed that small molecules and/or more water soluble molecules leak quite quickly, especially from microcapsules made by amine and aldehyde condensation reactions.
So whilst it is advantageous if the densities of microcapsules can be closely balanced to the density of the liquid product into which they are to be dispersed, this is particularly difficult to achieve for liquid products having densities greater than 1.010 g/cm3. Furthermore from the diverse constraints on the properties of microcapsule core materials and the need that the fragrance must be of sufficient quality to be acceptable in a premium commercial product, it is not immediately apparent that the density of the fragrance composition for a core shell microcapsule can be controlled to be close to or even greater than 0.950 g/cm3 just by formulating the contents from high density fragrance compounds.